Strange Self-Cloning Clam Steals Eggs From Other Clams

New research from the Brackenridge Field Laboratory at UT Austin is revealing the secret – and very strange – sexual practices of clams. Corbicula is an Asian freshwater clam that reproduces by cloning. They are physically hermaphroditic, meaning a clam possesses both male and female reproductive systems, but they are genetically male. A species of male hermaphroditic clones might sound bizarre, but it’s not completely unusual in the animal kingdom. What’s truly strange about this genus of clam is that every few hundred generations, it steals the eggs of other clams.

Most asexual species die out faster than sexual species, because their gene pool is limited. There's no way of introducing the genetic diversity that species need to keep going in the long run. But Corbicula has been going strong for 80 million years. UT scientists Shannon Hedtke and David Hillis wanted to know how they did it. So they scanned the genomes of 19 different Corbicula species, and found that their genes were all scrambled up. Genes that belonged to one species would show up in another. Here’s how Wired Science explains it:

According to Hillis and colleagues, the most plausible explanation involves rare fertilization events when sperm mets egg that doesn’t just come from another clam, but from another Corbicula species. Most of the time, development proceeds normally, with egg DNA jettisoned — but every so often, once in thousands or even millions of generations, some of the egg’s genes are allowed to stay. The clone’s lineage is replenished.

Corbicula is invasive, and it’s been found in Texas’ Colorado River. One clam can clone itself as many as 68,000 times per year, making the clams nearly impossible to get rid of.